Conventionally, electromagnetic actuators have been used as the drive source of acoustic elements such as speakers due to their ease of handling. An electromagnetic actuator is a component composed of a permanent magnet and a voice coil and that generates vibration due to the action of a magnetic circuit of a stator that uses the magnet. An electromagnetic speaker generates sound by means of vibration by a semi-rigid diaphragm such as organic film that is secured to the vibration part of the electromagnetic actuator. In recent years, the demand for compact and power-conserving electromagnetic actuators has increased with the increase in demand for portable telephones and notebook personal computers. In this regard, electromagnetic actuators are not only subject to problems regarding power conservation due to the need for the flow of a high current to the voice coil during operation, but are also not suited to more compact or slimmer construction due to their configuration. In addition, electromagnetic actuators require the application of an electromagnetic shield when applied to an electronic apparatus to prevent the adverse effects of magnetic flux leakage from the voice coil and, due to this factor as well, are not suited for use in a compact electronic apparatus such as a portable telephone. Still further, miniaturization brings about a reduction of the thickness of the wires of the voice coil, and the resulting increase of the resistance in the wire material raises the potential of burnout for the voice coil.
In view of the above-described problems, piezoelectric actuators have been developed as a thin vibration part to replace electromagnetic actuators, that take as a drive source a piezoelectric element such as a piezoelectric ceramic, and that have the characteristics of smaller size, light weight, power-saving capability, and the absence of magnetic flux leakage. A piezoelectric actuator is a component for generating mechanical vibration from the movement of a piezoelectric element, and has a configuration in which, for example, a piezoelectric ceramic element (also referred to as simply “piezoelectric element”) and a base are joined.
The basic configuration of a piezoelectric actuator is next described with reference to FIGS. 1 and 2. FIG. 1 is a perspective view showing the configuration of a piezoelectric actuator that relates to the present invention, and FIG. 2 is a sectional view giving a schematic representation of the state of the vibration of a piezoelectric actuator.
As shown in FIG. 1, piezoelectric actuator 550 includes: piezoelectric element 510 composed of a piezoelectric ceramic; base 524 to which piezoelectric element 510 is secured; and support member 527 having a frame shape for supporting the outer circumference of base 524. When an ac voltage is applied to piezoelectric element 510, piezoelectric element 510 moves by expanding and contracting. As shown in FIG. 2, base 524 changes to a convex mode (shown by the solid line) and to a concave mode (shown by the broken line) in accordance with the expanding and contracting movement. In this way, base 524 vibrates in the upward and downward directions of the figure with the base center as a diaphragm and junction part 524a as a fixed edge.
Although advantageous for helping to reduce the size of device, a piezoelectric actuator offers poorer acoustic performance as an acoustic element than an electromagnetic actuator. This degraded performance occurs because the high rigidity of the piezoelectric element itself results in a high mechanical Q value, whereby a vibration amplitude higher than an electromagnetic actuator can be obtained in the vicinity of the resonant frequency, but the vibration amplitude is smaller than an electromagnetic actuator in the band outside the resonant frequency. If the vibration amplitude of an actuator is low, the sound pressure is also low, which means that sufficient sound pressure cannot be obtained over the broad frequency band that is required for, for example, the reproduction of music. In response to this problem, JP-A S61-168971 and JP-A-2000-140759 disclose configurations in which the outer circumference of the base is supported by beams that are comparatively easy to deform to increase the vibration amplitude of the actuator.
In addition, JP-A-2001-17917 discloses a technique for obtaining large vibration amplitude in which, for a similar purpose, a leaf spring is formed in which slits are introduced along the circumference of the periphery of the base. Still further, JP-A-2001-339791 discloses a technique in which the base circumference and support member are joined by way of a curved support to broaden the frequency characteristic.